This project will use stable isotope tracers and combined gas chromatography-mass spectrometry to 1) study the physiology of amino acid transport and disposal, the conversion of essential to non-essential amino acds, and the fate of non-essential amino acid nitrogen in man; 2) to evaluate the effects of nutrition, energy balance, and aging on these rates; and 3) to investigate the pathology of these processes in diabetes and uremia, both disorders where protein synthesis is impaired and degradation accelerated. In the former entity, however, amino acid nitrogen is wasted in the urine while in the latter group, renal failure removes the effective route of nitrogen disposal. These results will be compared with correlates obtained under similar pathophysiological perturbations in the isolated rat epitrochlaris muscle. Taken together, these data should allow a more integrated picture of the quantitative dynamics of individual amino acid metabolism in man.